Supercritical fluid process for producing graphene dispersion from coke or coal

ABSTRACT

Provided is a process for producing isolated graphene sheets from a supply of coke or coal powder containing therein domains of hexagonal carbon atoms and/or hexagonal carbon atomic interlayers. The process comprises: (a) subjecting the supply of coke or coal powder to a supercritical fluid at a first temperature and a first pressure for a first period of time in a pressure vessel and then (b) rapidly depressurizing the supercritical fluid at a fluid release rate sufficient for effecting exfoliation and separation of the coke or coal powder to produce isolated graphene sheets, wherein the coke or coal powder is selected from petroleum coke, coal-derived coke, mesophase coke, synthetic coke, leonardite, anthracite, lignite coal, bituminous coal, or natural coal mineral powder, or a combination thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 15/211,408, filed Jul. 15, 2016, which is hereby incorporated byreference for all purposes.

FIELD OF THE INVENTION

The present invention relates to a process for producing isolated thingraphene sheets (single-layer or few-layer) directly from natural coalor coal derivatives (e.g. needle coke) using supercritical fluidintercalation, exfoliation, and separation.

BACKGROUND

A single-layer graphene sheet is composed of carbon atoms occupying atwo-dimensional hexagonal lattice. Multi-layer graphene is a plateletcomposed of more than one graphene plane. Individual single-layergraphene sheets and multi-layer graphene platelets are hereincollectively called nanographene platelets (NGPs) or graphene materials.NGPs include pristine graphene (essentially 99% of carbon atoms),slightly oxidized graphene (<5% by weight of oxygen), graphene oxide(≥5% by weight of oxygen), slightly fluorinated graphene (<5% by weightof fluorine), graphene fluoride ((≥5% by weight of fluorine), otherhalogenated graphene, and chemically functionalized graphene.

NGPs have been found to have a range of unusual physical, chemical, andmechanical properties. For instance, graphene was found to exhibit thehighest intrinsic strength and highest thermal conductivity of allexisting materials. Although practical electronic device applicationsfor graphene (e.g., replacing Si as a backbone in a transistor) are notenvisioned to occur within the next 5-10 years, its application as ananofiller in a composite material and an electrode material in energystorage devices is imminent. The availability of processable graphenesheets in large quantities is essential to the success in exploitingcomposite, energy, and other

Our research group was among the first to discover graphene [B. Z. Jangand W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patent applicationSer. No. 10/274,473, submitted on Oct. 21, 2002; now U.S. Pat. No.7,071,258 (Jul. 4, 2006)]. The processes for producing NGPs and NGPnanocomposites were reviewed by us [Bor Z. Jang and A. Zhamu,“Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: AReview,” J. Materials Sci. 43 (2008) 5092-5101]. Four main prior-artapproaches have been followed to produce NGPs. Their advantages andshortcomings are briefly summarized as follows:

Approach 1: Chemical Formation and Reduction of Graphite Oxide (GO)Platelets

The first approach entails treating natural graphite powder with anintercalant and an oxidant (e.g., concentrated sulfuric acid and nitricacid, respectively) to obtain a graphite intercalation compound (GIC)or, actually, graphite oxide (GO). [William S. Hummers, Jr., et al.,Preparation of Graphitic Oxide, Journal of the American ChemicalSociety, 1958, p. 1339.] Prior to intercalation or oxidation, graphitehas an inter-graphene plane spacing of approximately 0.335 nm (L_(d)=½d₀₀₂=0.335 nm). With an intercalation and oxidation treatment, theinter-graphene spacing is increased to a value typically greater than0.6 nm. This is the first expansion stage experienced by the graphitematerial during this chemical route. The obtained GIC or GO is thensubjected to further expansion (often referred to as exfoliation) usingeither a thermal shock exposure or a solution-based,ultrasonication-assisted graphene layer exfoliation approach.

In the thermal shock exposure approach, the GIC or GO is exposed to ahigh temperature (typically 800-1,050° C.) for a short period of time(typically 15 to 60 seconds) to exfoliate or expand the GIC or GO forthe formation of exfoliated or further expanded graphite, which istypically in the form of a “graphite worm” composed of graphite flakesthat are still interconnected with one another. This thermal shockprocedure can produce some separated graphite flakes or graphene sheets,but normally the majority of graphite flakes remain interconnected.Typically, the exfoliated graphite or graphite worm is then subjected toa flake separation treatment using air milling, mechanical shearing, orultrasonication in water. Hence, approach 1 basically entails threedistinct procedures: first expansion (oxidation or intercalation),further expansion (or “exfoliation”), and separation.

In the solution-based separation approach, the expanded (oxidized orintercalated) or exfoliated GO powder is dispersed in water or aqueousalcohol solution, which is subjected to ultrasonication. It is importantto note that in these processes, ultrasonifation is used afterintercalation and oxidation of graphite (i.e., after first expansion)and typically after thermal shock exposure of the resulting GIC or GO(after second expansion). Alternatively, the GO powder dispersed inwater is subjected to an ion exchange or lengthy purification procedurein such a manner that the repulsive forces between ions residing in theinter-planar spaces overcome the inter-graphene van der Waals forces,resulting in graphene layer separations.

There are several major problems associated with this conventionalchemical production process:

-   -   (1) The process requires the use of large quantities of several        undesirable chemicals, such as sulfuric acid, nitric acid, and        potassium permanganate or sodium chlorate.    -   (2) The chemical treatment process requires a long intercalation        and oxidation time, typically 5 hours to five days.    -   (3) Strong acids consume a significant amount of graphite during        this long intercalation or oxidation process by “eating their        way into the graphite” (converting graphite into carbon dioxide,        which is lost in the process). It is not unusual to lose 20-50%        by weight of the graphite material immersed in strong acids and        oxidizers.    -   (4) Both heat- and solution-induced exfoliation approaches        require a very tedious washing and purification step. For        instance, typically 2.5 kg of water is used to wash and recover        1 gram of GIC, producing huge quantities of waste water that        need to be properly treated.    -   (5) In both the heat- and solution-induced exfoliation        approaches, the resulting products are GO platelets that must        undergo a further chemical reduction treatment to reduce the        oxygen content. Typically even after reduction, the electrical        conductivity of GO platelets remains much lower than that of        pristine graphene. Furthermore, the reduction procedure often        involves the utilization of toxic chemicals, such as hydrazine.    -   (6) Furthermore, the quantity of intercalation solution retained        on the flakes after draining may range from 20 to 150 parts of        solution by weight per 100 parts by weight of graphite flakes        (pph) and more typically about 50 to 120 pph.    -   (7) During the high-temperature exfoliation, the residual        intercalant species (e.g. sulfuric acid and nitric acid)        retained by the flakes decompose to produce various species of        sulfuric and nitrous compounds (e.g., NO_(x) and SO_(x)), which        are undesirable. The effluents require expensive remediation        procedures in order not to have an adverse environmental impact.        The present invention was made to overcome the limitations        outlined above.        Approach 2: Direct Formation of Pristine Nanographene Sheets

In 2002, our research team succeeded in isolating single-layer andmulti-layer graphene sheets from partially carbonized or graphitizedpolymeric carbons, which were obtained from a polymer or pitch precursor[B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/274,473, submitted on Oct. 21, 2002; now U.S.Pat. No. 7,071,258 (Jul. 4, 2006)]. Mack, et al [“Chemical manufactureof nanostructured materials” U.S. Pat. No. 6,872,330 (Mar. 29, 2005)]developed a process that involved intercalating natural graphite withpotassium metal melt and contacting the resulting K-intercalatedgraphite with alcohol, producing violently exfoliated graphitecontaining NGPs. The process must be carefully conducted in a vacuum oran extremely dry glove box environment since pure alkali metals, such aspotassium and sodium, are extremely sensitive to moisture and pose anexplosion danger. This process is not amenable to the mass production ofNGPs. The present invention was made to overcome the limitationsoutlined above.

Approach 3: Epitaxial Growth and Chemical Vapor Deposition ofNanographene Sheets on Inorganic Crystal Surfaces

Small-scale production of ultra-thin graphene sheets on a substrate canbe obtained by thermal decomposition-based epitaxial growth and a laserdesorption-ionization technique. [Walt A. DeHeer, Claire Berger, PhillipN. First, “Patterned thin film graphite devices and method for makingsame” U.S. Pat. No. 7,327,000 B2 (Jun. 12, 2003)] Epitaxial films ofgraphite with only one or a few atomic layers are of technological andscientific significance due to their peculiar characteristics and greatpotential as a device substrate. However, these processes are notsuitable for mass production of isolated graphene sheets for compositematerials and energy storage applications.

Approach 4: The Bottom-Up Approach (Synthesis of Graphene from SmallMolecules)

Yang, et al. [“Two-dimensional Graphene Nano-ribbons,” J. Am. Chem. Soc.130 (2008) 4216-17] synthesized nanographene sheets with lengths of upto 12 nm using a method that began with Suzuki-Miyaura coupling of1,4-diiodo-2,3,5,6-tetraphenyl-benzene with 4-bromophenylboronic acid.The resulting hexaphenylbenzene derivative was further derivatized andring-fused into small graphene sheets. This is a slow process that thusfar has produced very small graphene sheets.

Hence, an urgent need exists to have a graphene production process thatrequires a reduced amount of undesirable chemicals (or elimination ofthese chemicals all together), shortened process time, less energyconsumption, lower degree of graphene oxidation, reduced or eliminatedeffluents of undesirable chemical species into the drainage (e.g.,sulfuric acid) or into the air (e.g., SO₂ and NO₂). The process shouldbe able to produce more pristine (less oxidized and less damaged), moreelectrically conductive, and larger/wider graphene sheets.

Furthermore, most of the prior art processes for graphene productionbegin with the use of highly purified natural graphite as the startingmaterial. The purification of graphite ore involves the use of largeamounts of undesirable chemicals. Clearly, a need exists to have a morecost-effective process that produces graphene sheets (particularlysingle-layer graphene and few-layer graphene sheets) directly from coalor coal derivatives. Such a process not only avoids theenvironment-polluting graphite ore purification procedures but alsomakes it possible to have low-cost graphene available. As of today, thegraphene, as an industry, has yet to emerge mainly due to the extremelyhigh graphene costs that have thus far prohibited graphene-basedproducts from being widely accepted in the marketplace.

SUMMARY OF THE INVENTION

The present invention provides a process for producing isolated graphenesheets having an average thickness smaller than 10 nm (preferably andtypically single-layer graphene or few-layer graphene) directly from acoke or coal powder having hexagonal carbon atomic interlayers ordomains (graphene planes or graphene domains) with an interlayer spacing(inter-graphene plane spacing).

The process for producing isolated graphene sheets from a supply of cokeor coal powder comprises: (a) bringing this supply of coke or coalpowder in contact with a supercritical fluid at a first temperature anda first pressure for a first period of time in a pressure vessel(preferably completely immersing the coal/coke powder in this fluid) toenable penetration of the supercritical fluid into the internalstructure of the coke/coal particles, and then (b) rapidlydepressurizing the supercritical fluid at a fluid release ratesufficient for effecting exfoliation and separation of the coke or coalpowder to produce the isolated graphene sheets; wherein the coke or coalpowder is selected from petroleum coke, coal-derived coke, mesophasecoke, synthetic coke, leonardite, anthracite, lignite coal, bituminouscoal, or natural coal mineral powder, or a combination thereof. Incertain embodiments, these particles of coke or coal powder have neverbeen previously intercalated or oxidized prior to step (a).

The supercritical fluid as used herein can comprise a fluid selectedfrom carbon dioxide, water, hydrogen peroxide (H₂O₂), methanol, ethanol,acetone, methane, ethane, propane, ethylene, propylene, nitrous oxide(N₂O), ozone, sulfonic group (SO₃), or a combination thereof.

In some preferred embodiments, step (a) of the process is conductedunder the influence of ultrasonic waves. In other words, the coke/coalpowder is subjected to concurrent treatments by ultrasonication andsupercritical fluid penetration/intercalation in the same pressurechamber.

The depressurizing step may be followed by a mechanical shearingtreatment selected from air milling, air jet milling, wet milling, ballmilling, rotating blade shearing, ultrasonication, or a combinationthereof. This mechanical shearing treatment may be used to furtherreduce the thickness (number of graphene planes) of isolated graphenesheets. This may be conducted when the resulting graphene sheets afterthe supercritical fluid treatment are multi-layer graphene platelets(from 2 to 20 layers).

Alternatively, multi-layer graphene platelets may be reduced tofew-layer (2-10 planes) or single-layer graphene by repeating thesupercritical fluid exposure and de-pressurization treatments. Thus, insome embodiments, the process further comprises a procedure ofessentially repeating step (a) and step (b) that includes (i) subjectingthe isolated graphene sheets to a supercritical fluid at a secondtemperature and a second pressure for a second period of time in apressure vessel and then (ii) rapidly depressurizing the fluid at afluid release rate sufficient for effecting further exfoliation andseparation of graphene sheets. The second temperature may be the same ordifferent from the first temperature and the second pressure may be thesame or different from the first pressure.

Preferably, the supercritical fluid contains a surfactant or dispersingagent dissolved therein. Thus, the pressure vessel may further contain asurfactant or dispersing agent selected from the group consisting ofanionic surfactants, nonionic surfactants, cationic surfactants,amphoteric surfactants, silicone surfactants, fluoro-surfactants,polymeric surfactants, sodium hexametaphosphate, sodium lignosulphonate,poly (sodium 4-styrene sulfonate), sodium dodecylsulfate, sodiumsulfate, sodium phosphate, sodium sulfonate, and combinations thereof.

In some embodiments, the pressure vessel further contains a surfactantor dispersing agent selected from melamine, ammonium sulfate, sodiumdodecyl sulfate, sodium (ethylenediamine), tetraalkylammonium, ammonia,carb amide, hexamethylenetetramine, organic amine, pyrene,1-pyrenecarboxylic acid, 1-pyrenebutyric acid, 1-pyrenamine,poly(sodium-4-styrene sulfonate), or a combination thereof.

In certain embodiments, the supercritical fluid contains therein anorganic solvent, a monomer, an oligomer, a polymer solution, or acombination thereof. In some embodiments, the supercritical fluidfurther comprises a monomer or an oligomer dispersed in the fluid andstep (b) of the process is followed by polymerization of the monomer oroligomer to form a polymer.

In certain embodiments, the supercritical fluid contains a coating agentdissolved therein. The coating agent may comprise a monomer, aprepolymer or oligomer, a polymer, a resin, a curing agent, or acombination thereof.

In some embodiments, the supercritical fluid contains a curing agentdissolved therein and the process further comprises mixing the isolatedgraphene sheets with a thermosetting resin.

In the invented process, the supercritical fluid may comprise a reactivechemical group and step (a) imparts the reactive chemical group to theisolated graphene sheets.

The invented process may be conducted intermittently or continuously andthe supply of coke or coal powder and supercritical liquid are providedinto the pressure vessel intermittently or continuously.

In some embodiments, the supercritical fluid contains a curing agentdissolved therein and the process further comprises mixing the isolatedgraphene sheets with a thermosetting resin to form a mixture.Preferably, the process further comprises a step of curing thegraphene-resin mixture into a composite material.

The present invention also provides a graphene-containing inkcomposition containing graphene sheets derived from coal or cokeproduced by the presently invented process. Also provided is a compositematerial containing graphene sheets produced by the invented process.The invention also provides a mass of isolated graphene sheets in apowder form produced in the invented process. Also provided is asuspension containing a liquid medium and isolated graphene sheetsproduced by the invented process and these graphene sheets are dispersedin this liquid medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A flow chart showing an embodiment of the presently inventedprocess for producing isolated graphene sheets.

FIG. 2 Schematic drawing of an apparatus that submits coal/coke slurryto a supercritical fluid to produce isolated graphene sheets.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Carbon materials can assume an essentially amorphous structure (glassycarbon), a highly organized crystal (graphite), or a whole range ofintermediate structures that are characterized in that variousproportions and sizes of graphite crystallites and defects are dispersedin an amorphous matrix. Typically, a graphite crystallite is composed ofa number of graphene sheets or basal planes that are bonded togetherthrough van der Waals forces in the c-axis direction, the directionperpendicular to the basal plane. These graphite crystallites aretypically micron- or nanometer-sized. The graphite crystallites aredispersed in or connected by crystal defects or an amorphous phase in agraphite particle, which can be a graphite flake, carbon/graphite fibersegment, carbon/graphite whisker, or carbon/graphite nanofiber. In thecase of a carbon or graphite fiber segment, the graphene plates may be apart of a characteristic “turbostratic structure.”

Basically, a graphite material is composed of many graphene planes(hexagonal carbon atomic interlayers) stacked together havinginter-planar spacing. These graphene planes can be exfoliated andseparated to obtain isolated graphene sheets that can each contain onegraphene plane or several graphene planes of hexagonal carbon atoms.Further, natural graphite refers to a graphite material that is producedthrough purification of graphite mineral (mined graphite ore or graphiterock) typically by a series of flotation and acid treatments. Particlesof natural graphite are then subjected to intercalation/oxidation,expansion/exfoliation, and separation/isolation treatments as discussedin the Background section.

The instant invention obviates the need to go through the graphitepurification procedures that otherwise generate great amounts ofpolluting chemicals. In fact, the instant invention avoids the use ofnatural graphite all together as a starting material for the productionof graphene sheets. Instead, we begin with coal or its derivatives(including coke, particularly needle coke). No undesirable chemicals,such as concentrated sulfuric acid, nitric acid, and potassiumpermanganate, are used in the presently invented method.

One preferred specific embodiment of the present invention is a processfor producing isolated graphene sheets, also called nanographeneplatelets (NGPs), directly from coal powder without purification. Wehave surprisingly discovered that powder of coal (e.g. leonardite orlignite coal) contains therein graphene-like domains or stacks ofaromatic molecules that span from 5 nm to 1 μm in length or width. Thesegraphene-like domains contain planes of hexagonal carbon atoms and/orhexagonal carbon atomic interlayers with an interlayer spacing. Thesestacks or domains of graphene-like planes, molecules or interlayers aretypically interconnected with disordered chemical groups containingtypically C, O, N, P, and/or H. The presently invented supercriticalfluid process is capable of intercalating, exfoliating, and/orseparating the interlayers and/or separating domains of graphene-likeplanes from the surrounding disordered chemical species to obtainisolated graphene sheets.

Each graphene sheet comprises one or multiple planes of two-dimensionalhexagonal structure of carbon atoms. Each graphene sheet has a lengthand a width parallel to the graphene plane and a thickness orthogonal tothe graphene plane. By definition, the thickness of an NGP is 100nanometers (nm) or smaller (more typically <10 nm and most typically anddesirably <3.4 nm (i.e. few-layer graphene sheets each containing 2-10graphene planes), with a single-sheet NGP (single-layer graphene) beingas thin as 0.34 nm. The length and width of a NGP are typically between5 nm and 10 μm, but could be longer or shorter. Generally, the graphenesheets produced from the coal or coke powder using the presentlyinvented method are single-layer graphene or few-layer graphene (2-10graphene planes stacked together).

A preferred embodiment of the present invention is a process thatinvolves intercalating, exfoliating, and/or separating a coal or cokematerial with a supercritical fluid to obtain isolated graphene sheets.Optionally, a supercritical fluid can comprise therein a surfactant (ordispersing agent), a coating agent (e.g., a monomer, curing agent, orresin), and/or a reactive species (e.g., ozone, oxygen, acid vapor, SO₃,etc.).

If a substance is heated above a critical temperature (Tc) andpressurized above a critical pressure (Pc), it becomes a supercriticalfluid. Supercritical fluids provide convenient means to achievesolvating properties, which have both gas and liquid characteristicswithout actually changing the chemical structure of a substance. With acareful control over the pressure and temperature, severalphysicochemical properties (e.g., density, diffusivity, dielectricconstant, viscosity, and surface free energy) of this substance can bevaried to a significant extent. In particular, under supercritical fluidconditions, a fluid can readily diffuse into the internal structure of asolid material. We have surprisingly observed that supercritical fluidscan penetrate spaces between graphene-like or graphene oxide-like planesin a coal or coke structure and, through variations in temperature andpressure can exfoliate and separate these planes to obtain isolatedgraphene sheets. The supercritical fluids are also surprisingly capableof severing and extracting graphene domains or individual graphenesheets from a complex graphene domain-amorphous structure commonly foundin a coal or coke material.

As an example, carbon dioxide may exist as a supercritical fluid havingproperties of both a liquid and a gas when above its criticaltemperature (>31° C.) and critical pressure (>7.4 MPa). Undersupercritical conditions, CO₂ exhibits both a gaseous property, beingable to penetrate through many materials, and a liquid property, beingable to dissolve many materials. Although CO₂ is a preferred medium, thesupercritical fluid may be selected from other suitable species, such aswater, hydrogen peroxide, ozone, nitrous oxide, methane, ethane,ethylene, propylene, ethanol, methanol, or a mixture thereof (Table 1).The supercritical point of water comprises a temperature of at leastabout 374° C. and a pressure of at least about 22.1 MPa. At or about thesupercritical point, the density and viscosity of water decreases andthe diffusivity of water molecules and the mobility of other chemicalspecies dissolved in the water increase.

TABLE 1 Critical properties of various solvents (Reid, R. C.; Prausnitz,J. M. and Poling, B. E., The Properties of Gases and Liquids,McGraw-Hill, New York, 987). Molec- Critical ular temper- CriticalCritical weight ature (Tc) pressure (Pc) density Solvent g/mol ° K. MPa(atm) g/cm³ Carbon dioxide 44.01 304.1 7.38 (72.8) 0.469 (CO₂) Water(H₂O) 18.015 647.096  22.064 (217.755) 0.322 Methane (CH₄) 16.04 190.44.60 (45.4) 0.162 Ethane (C₂H₆) 30.07 305.3 4.87 (48.1) 0.203 Propane(C₃H₈) 44.09 369.8 4.25 (41.9) 0.217 Ethylene (C₂H₄) 28.05 282.4 5.04(49.7) 0.215 Propylene (C₃H₆) 42.08 364.9 4.60 (45.4) 0.232 Methanol(CH₃OH) 32.04 512.6 8.09 (79.8) 0.272 Ethanol (C₂H₅OH) 46.07 513.9 6.14(60.6) 0.276 Acetone (C₃H₆O) 58.08 508.1 4.70 (46.4) 0.278 Nitrous oxide44.013 306.57 7.35 (72.5) 0.452 (N₂O)

Hence, as a preferred embodiment, the presently invented processcomprises: (a) exposing a coal or coke material to a supercritical fluidat a first temperature and a first pressure for a first period of timein a pressure vessel and then (b) rapidly depressurizing the fluid bydischarging part of the fluid out of the vessel at a fluid release ratesufficient for effecting exfoliation and separation of the graphene-likedomains to obtain the desired graphene sheets. Presumably, thesupercritical fluid in the pressure vessel penetrates into the spacesbetween graphene-like planes or between graphene-like domains to form atentatively intercalated coal/coke compound. With rapiddepressurization, the fluid in the interstitial spaces or betweendomains quickly expands to push apart neighboring graphene layers orsevering the amorphous carbon regions that connect graphene-like domainstogether, a process called delamination/exfoliation orseparation/isolation. This step produces graphene sheets that aretypically thinner than 10 nm in thickness, more typically thinner than3.4 nm (few-layer graphene), and often single-layer graphene. Thestarting material may be selected from petroleum coke, coal-derivedcoke, mesophase coke, synthetic coke, leonardite, anthracite, lignitecoal, bituminous coal, or natural coal mineral powder, or a combinationthereof.

As a first step, the starting material (e.g., powder of needle coke orlignite coal) is placed inside a high pressure vessel. The vessel isthen sealed off from the atmosphere. This is followed by introducinghigh-pressure CO₂ into the vessel with CO₂ being pressurized topreferably above approximately 1,070 to 10,000 psig (7.4 MPa to 69 MPa).Then, the vessel is heated to a temperature above 31.5° C., preferablyabove about 40° C., and more preferably above 70° C. These conditionsdefine a supercritical condition of CO₂ whereby the CO₂ is capable ofpenetrating into inter-graphene spaces or between graphene domains.Pressurizing and heating the coal or coke particles with thesupercritical fluid may be accomplished by any conventional means. Forinstance, the vessel may be heated by a heating jacket or electricalheating tape wrapped around the vessel.

If a chemical species, such as reactive ozone molecules, is desired, itcan be introduced into the pressure vessel before, during, or after theintended supercritical fluid is introduced. If the species is in aliquid state (e.g., a surfactant or a curing agent for a resin) or solidstate (e.g., a resin), it is preferably placed into the vessel (e.g.,mixed with the starting coal/coke material) prior to sealing off thevessel.

The procedure further comprises rapidly depressurizing the tentativelyintercalated coal/coke by releasing the fluid out of the vessel at ahigh rate. During catastrophic depressurization, the supercritical fluidrapidly expands to exfoliate/separate the graphene planes or domains.The depressurization step comprises immediately depressurizing thevessel down to a considerably lower pressure, preferably ambientpressure. This may be accomplished in a time period of between about 5and 30 seconds, and preferably 15 seconds. Specifically, this may beaccomplished by depressurizing the pressure vessel at a rate of betweenabout 0.1 and 5.0 milliliters per second, and preferably 3.0 millilitersper second. The pressure decrease may be accomplished by opening avessel valve to the atmosphere. As immediate depressurization occurs,the graphite layers are delaminated apart from one another.

The process may further comprise a procedure that involves essentiallyrepeating the pressurization/heating step and the depressurization stepfor at least another cycle. The repeating cycle includes (a) exposingthe graphene sheets (containing multi-layer sheets) to a supercriticalfluid at a second temperature and a second pressure for a second periodof time in a pressure vessel (preferably the same vessel) and then (b)rapidly depressurizing the fluid at a fluid release rate sufficient foreffecting further exfoliation/separation of the graphene planes. Thesecond temperature may be different from or the same as the firsttemperature and the second pressure may be different from or the same asthe first pressure. It was observed that a higher pressure for a givenpressurization time tended to result in a more effectiveexfoliation/separation, as evidenced by a reduced average graphenethickness.

In another preferred embodiment, the supercritical fluid contains asurfactant or dispersing agent dissolved therein. Surfactants ordispersing agents that can be used include anionic surfactants,non-ionic surfactants, cationic surfactants, amphoteric surfactants,silicone surfactants, fluoro-surfactants, and polymeric surfactants.Particularly useful surfactants for practicing the present inventioninclude DuPont's Zonyl series that entails anionic, cationic, non-ionic,and fluoro-based species. Other useful dispersing agents include sodiumhexameta-phosphate, sodium lignosulphonate (e.g., marketed under thetrade names Vanisperse CB and Marasperse CBOS-4 from BorregaardLignoTech), sodium sulfate, sodium phosphate, and sodium sulfonate.Presumably, a surfactant is capable of rapidly covering the new surfacescreated during the delamination or separation between two graphenelayers, thereby preventing the re-formation of inter-graphene van derWaals forces (re-stacking of two graphene sheets). This speculation wasconsistent with our surprising observation that the presence of asurfactant tends to result in much thinner graphene sheets as comparedwith the surfactant-free case under comparable processing conditions. Itis of significance to note that the surfactant is normally easy toremove after the formation of isolated graphene sheets; e.g., viaheat-induced vaporization or simple water rinsing.

Again, the procedure of supercritical fluid intercalation (pressurizingand heating) and exfoliation/separation (depressurization) can berepeated for at least another cycle to further reduce the thickness ofNGPs. The cycle can include (a) subjecting the graphene sheets to asupercritical fluid (containing a surfactant dissolved therein) at asecond temperature and a second pressure for a second period of time ina pressure vessel and then (b) rapidly depressurizing the fluid at afluid release rate sufficient for effecting further exfoliation of theNGP material. Again, the second temperature may be different from or thesame as the first temperature and the second pressure may be differentfrom or the same as the first pressure.

In yet another preferred embodiment of the present invention, thesupercritical fluid contains a coating agent dissolved therein. Theprocess comprises supercritical fluid intercalation (at a firsttemperature and first pressure) and exfoliation/separation of acoal/coke material to produce graphene sheets and then repeating thesupercritical fluid intercalation and exfoliation/separation steps forthe resulting graphene sheets. These repeating steps include (a)subjecting the graphene sheets to a supercritical fluid (containing acoating agent dissolved therein) at a second temperature and a secondpressure for a second period of time in a pressure vessel and then (b)rapidly de-pressurizing the fluid at a fluid release rate sufficient foreffecting further exfoliation/separation of the graphene sheets.

The coating agent may comprise a monomer, a prepolymer or oligomer, apolymer, a resin, a curing agent, or a combination thereof. This processis particularly useful for the production of thin NGP-reinforced polymercomposites. For the preparation of a thermoset resin composite, it isadvantageous to have a supercritical fluid containing a curing agentdissolved therein. The curing agent, typically a low molecular weightspecies, can penetrate into the inter-graphene spaces (also referred toas interstitial spaces), along with the supercritical fluid.Upon-depressurization, the curing agent will precipitate out to coverthe newly formed graphene surfaces. In addition to possibly serving toprevent the re-joining of graphene layers, the curing agent also acts tochange the graphene surface properties, promoting the subsequent wettingof the graphene surface by a thermosetting resin (e.g., epoxide). Hence,the process further comprises mixing the curing agent-covered graphenematerial with a thermosetting resin.

One may choose to use a coating agent that can be solubilized in thesupercritical fluid to diffuse between the graphene planes or betweentwo neighboring graphene-like domains. This coating agent could expandor swell the interstitial spaces between graphene planes to assist inintercalation and exfoliation and, after depressurization, the coatingagent could precipitate out to surround and isolate the exfoliated orseparated graphene sheets. This coating agent (e.g., a polymer) willeventually become a part (e.g. the matrix) of a composite material.Generally, the coating agent may include a polymer, oligomer,prepolymer, or a monomer. In one embodiment, the coating agent ispoly-(dimethyl siloxane) (“PDMS”) having a weight average molecularweight of preferably between about 30,000 and 200,000 g/mole. Othersuitable coating agents includepoly-(tetrafluoroethylene-co-hexafluoropropylene),poly-(perfluoro-propylene oxide), poly-(diethyl-siloxane),poly-(dimethyl silicone), poly-(phenylmethylsilicone),perfluoroalkylpolyethers, chloro-trifluoro-ethylene, andbromotrifluoroethylene.

The coal/coke powder particles and the coating agent may be disposed ina high pressure vessel that is isolatable from the atmosphere. In thisembodiment, the coal/coke particles comprise about 25 to 85 weightpercent and the coating agent comprises about 15 to 75 weight percent ofmaterial placed in the vessel. Then, the pressure vessel is sealed offfrom the atmosphere. This is followed by introducing high-pressurecarbon dioxide into the compartment with CO₂ being pressurized in thevessel to preferably above approximately 1,070 psig (7.4 MPa). Then, thevessel is heated to a temperature preferably above about 40° C. Theseconditions define a supercritical condition of carbon dioxide wherebythe coating agent is solubilized in the supercritical carbon dioxide.

With the coating agent being solubilized in the supercritical fluid, thecoating agent diffuses into inter-graphene spaces to possibly expand orswell these spaces. The step of diffusing the coating agent into thespaces between the graphene planes includes maintaining diffusion forbetween about 10 minutes to 24 hours (preferably 1-3 hours) atsupercritical conditions to produce tentatively intercalated coal/coke.The procedure further comprises catastrophically depressurizing thetentatively intercalated coal/coke to precipitate the coating agent fromthe supercritical fluid. During catastrophic depressurization, thesupercritical fluid expands and exfoliates the graphene planes while thecoating agent precipitates from the supercritical fluid to cover thelayers. Although a coating agent could help, but we have discovered thattypically the supercritical fluid alone is sufficiently effective inexfoliating/separating graphene sheets from coal/coke powder.

Presumably, the low viscosity and high diffusivity of the supercriticalfluid allows the coating agent solubilized therein to becomeintercalated between the graphene planes in the coal/coke material undersupercritical conditions, thereby possibly increasing the interlayerspacing. Upon depressurization, the supercritical fluid residing in theinterstitial spaces rapidly expand and force the layers to exfoliate ordelaminate from each other, and the coating agent previously solubilizedin the supercritical fluid precipitates therefrom to deposit on thedelaminated layers, preventing reformation of the van der Waals forcesbetween graphene layers. That is, the coating agent precipitates fromthe supercritical fluid and attaches itself to the graphene sheetsurfaces.

Although this conventional route is useful in terms of producingpristine graphene sheets that are covered with a coating agent, one hasto remove this coating agent unless the coating agent is desired for anintended application (e.g., for the preparation of a polymer matrixcomposite with the coating agent being the monomer or polymer for thismatrix). For this particular purpose, it is advantageous to have asupercritical fluid containing a curing agent dissolved therein. Thecuring agent, typically a low molecular weight species, can penetrateinto the inter-graphene spaces, along with the supercritical fluid.Upon-depressurization, the curing agent will precipitate out to coverthe newly formed graphene surfaces. In addition to possibly acting toprevent the re-stacking of graphene layers, the curing agent also servesto change the graphene surface properties, promoting the subsequentwetting of the graphene surface by a thermosetting resin (e.g.,epoxide). Hence, the process further comprises mixing the curingagent-covered graphene material with a thermosetting resin.

-   -   In summary, after an extensive study, we have surprisingly        observed that:    -   (1) Supercritical fluids containing no coating agent are in        general as effective as those containing a coating agent for        intercalating, exfoliating, and separating coal/coke powder.        There is no major difference in the supercritical fluid        temperature, pressure, time, and de-pressurization conditions        between the two cases (one with and the other without a coating        agent);    -   (2) Supercritical fluids, with or without a coating agent        dissolved therein, are effective in intercalating, exfoliating,        and separating a wide variety of coal/coke materials, including        from petroleum coke, coal-derived coke, mesophase coke,        synthetic coke, leonardite, anthracite, lignite coal, bituminous        coal, and natural coal mineral powder.    -   (3) With proper conditions selected for supercritical fluid        intercalation and exfoliation, one could readily obtain        ultra-thin graphene sheets with a thickness less than 1 nm. With        other less favorable conditions (e.g., a lower depressurization        rate or gas discharge rate), somewhat thicker NGPs were        obtained. However, these thicker NGPs could be subjected to        another cycle of supercritical fluid intercalation and        exfoliation, preferably in the same pressure chamber, to yield        much thinner NGPs. By repeating the cycle one or two times we        could readily obtain substantially single-layer graphene.    -   (4) Supercritical fluids containing a surfactant dissolved        therein are more effective than their counterparts containing a        coating agent (e.g., polymer, monomer, and oil) or those        containing no surfactant and no coating agent.    -   (5) The presently invented process is fast and environmentally        benign.    -   (6) A functional group can be conveniently imparted to the        resulting NGPs if a reactive chemical group is introduced into        the pressure vessel to contact the NGPs therein before, during,        or after NGPs are formed.

The best modes practice of instant invention are discussed in moredetails as follows: A preferred embodiment of the invention isschematically illustrated in FIG. 1 and FIG. 2. The process comprisestwo steps. Step (a) entails supplying particles of coke or coal powderand an optional surfactant, dispersing agent, or coating agent into apressure chamber (30 in FIG. 2) and introducing a pressurized fluid intothe chamber. The fluid is then heated and pressurized to reach asupercritical fluid state (e.g. a temperature of >31.1° C. and pressureof >72.8 atm for CO₂), wherein the optional surfactant, dispersingagent, or coating agent is dissolved in the supercritical fluid (32 inFIG. 2). The coke or coal powder may be selected from petroleum coke,coal-derived coke, mesophase coke, synthetic coke, leonardite,anthracite, lignite coal, bituminous coal, or natural coal mineralpowder, or a combination thereof.

Step (b) entails rapidly de-pressurizing the pressure vessel to producethe isolated graphene sheets. Under certain supercritical fluidconditions (e.g. at the lower end of the pressure scale), the producedgraphene sheets can contain some multi-layer graphene platelets orsheets, each typically containing from 5 to 20 graphene planes. Thesegraphene sheets may stay in the same pressure chamber or be dischargedinto a different pressure chamber having a pressurized fluid, which isthen re-pressurized and heated to reach a supercritical fluid condition.After a desired period of supercritical fluid exposure time, the chamberis rapidly de-pressurized to produce thinner graphene sheets. The sameor similar pressurization and de-pressurization procedures may berepeated until a desired average graphene thickness (e.g. all graphenesheets being single-layer graphene) is reached. A cascade ofsupercritical fluid chambers may be connected in series.

As shown in FIG. 2, the pressure chamber (vessel 30) may be equippedwith a fluid inlet 36 for introducing pressurized fluid into the vesseland a fluid outlet 38 for rapidly releasing the de-pressurized fluid,which can be re-cycled and re-used. A safety valve is also provided toregulate the fluid pressure of the vessel. The coal or coke powder maybe charged, on demand, into the pressure chamber 30 through a controlledconduit 34 and the processed graphene sheets may be discharged through adischarge chamber 40 and get collected by a bag or drop into acollecting liquid.

Alternatively, thick graphene sheets/platelets, after first or secondpressurization/de-pressurization treatment, may be subjected tomechanical separation treatments (e.g. airjet milling, rotating-bladeshearing, wet milling, etc.) to obtain thinner graphene sheets.

Further alternatively, the pressure vessel may be equipped with anultrasonicator device, subjecting the supercritical fluid and thecoal/coke powder dispersed therein to ultrasonication. Exposure ofcoal/coke powder to concurrent supercritical fluid and ultrasonicationtreatments can produce essentially all few-layer or all single-layergraphene sheets.

Using needle coke as an example, the first step may involve preparing acoke powder sample containing fine needle coke particulates(needle-shaped). The length and/or diameter of these particles arepreferably less than 0.2 mm (<200 μm), further preferably less than 0.01mm (10 μm). They can be smaller than 1 μm. The needle coke particlestypically contain nanometer-scaled graphite crystallites with eachcrystallite being composed of multiple graphene planes.

In one example, the coke powder is then dispersed in a liquid medium(e.g., water, alcohol, or acetone) to obtain a suspension inside apressure vessel, which is then pressurized and heated for the liquidmedium to reach a supercritical fluid state (e.g. 513° K and 8.1 MPa formethanol). A dispersing agent or surfactant may be used to helpuniformly disperse particles in the liquid medium. Most importantly, wehave surprisingly found that the dispersing agent or surfactantfacilitates the exfoliation and separation of graphene sheets fromcoal/coke particles. Under comparable processing conditions, a coke/coalsample containing a surfactant usually results in much thinner graphenesheets compared to a sample containing no surfactant. It also takes ashorter length of pressurization time for a surfactant-containingsuspension to achieve a desired platelet dimension.

Surfactants or dispersing agents that can be used include anionicsurfactants, non-ionic surfactants, cationic surfactants, amphotericsurfactants, silicone surfactants, fluoro-surfactants, and polymericsurfactants. Particularly useful surfactants for practicing the presentinvention include DuPont's Zonyl series that entails anionic, cationic,non-ionic, and fluoro-based species. Other useful dispersing agentsinclude sodium hexametaphosphate, sodium lignosulphonate (e.g., marketedunder the trade names Vanisperse CB and Marasperse CBOS-4 fromBorregaard LignoTech), sodium sulfate, sodium phosphate, and sodiumsulfonate.

Advantageously, the surfactant or dispersing agent may be selected frommelamine, ammonium sulfate, sodium dodecyl sulfate, sodium(ethylenediamine), tetraalkylammonium, ammonia, carbamide,hexamethylenetetramine, organic amine, pyrene, 1-pyrenecarboxylic acid,1-pyrenebutyric acid, 1-pyrenamine, poly(sodium-4-styrene sulfonate), ora combination thereof.

It may be noted that the conventional process for the formation ofgraphite intercalation compound (GICs) involves the use of highlyoxidizing agents (e.g. nitric acid or potassium permanganate), whichcauses severe oxidation to graphite. Upon oxidation, graphite wouldsuffer from a dramatic loss in electrical and thermal conductivity andthis normally cannot be fully recovered.

In contrast, the presently invented process makes use of only very mildfluid mediums (water, alcohol, etc.). Hence, this process obviates theneed or possibility to expose the coke/coal material to an oxidizingenvironment. If so desired, the product after supercritical fluidtreatment may be subjected to a subsequent mechanical shearingtreatment, such as ball milling, air milling, or rotating-bladeshearing, at a relatively low temperature (e.g., room temperature). Withthis treatment, either individual graphene planes or stacks of grapheneplanes bonded together (multi-layer NGPs) are further reduced inthickness (decreasing number of layers), width, and length. In additionto the thickness dimension being nanoscaled, both the length and widthof these NGPs could be reduced to smaller than 100 nm in size if sodesired.

The exfoliation step in the instant invention does not involve theevolution of undesirable species, such as NO_(x) and SO_(x), which arecommon by-products of exfoliating conventional sulfuric or nitricacid-intercalated graphite compounds. These chemical species are highlyregulated worldwide.

Supercritical fluid state also enables the resulting graphene sheets tobe well dispersed in the very liquid medium wherein the coke/coal powderis dispersed, producing a homogeneous suspension. One major advantage ofthis approach is that exfoliation, separation, and dispersion ofgraphene sheets are achieved in a single step. A monomer, oligomer, orpolymer may be added to this suspension to form a suspension that is aprecursor to a nanocomposite structure. The process may include afurther step of converting the suspension to a mat or paper (e.g., usingany well-known paper-making process), or converting the nanocompositeprecursor suspension to a nanocomposite solid.

Thus, in certain embodiments, the liquid medium comprises water, organicsolvent, alcohol, a monomer, an oligomer, or a combination thereof. Inother embodiments, the liquid medium further comprises a monomer or anoligomer dispersed in the liquid medium and step (b) is followed bypolymerization of the monomer or oligomer to form a polymer. Thegraphene sheets concurrently produced can be well-dispersed in thepolymer. This added advantage is also unexpected.

In some embodiments of the invention, the liquid medium furthercomprises a polymer dissolved or dispersed in the liquid medium and theisolated graphene sheets are mixed with the polymer to form a compositecomposition. This is a good approach to the preparation ofgraphene-reinforced polymer composites.

Alternatively, the resulting graphene sheets, after drying to become asolid powder, may be mixed with a monomer to form a mixture, which canbe polymerized to obtain a nanocomposite solid. The graphene sheets canbe mixed with a polymer melt to form a mixture that is subsequentlysolidified to become a nanocomposite solid.

Again, a coating agent for adding into the supercritical fluid may beselected from melamine, ammonium sulfate, sodium dodecyl sulfate, sodium(ethylenediamine), tetraalkylammonium, ammonia, carbamide,hexamethylenetetramine, organic amine, poly(sodium-4-styrene sulfonate),or a combination thereof. Some of the wetting agents (e.g. thosecontaining an amine group) also serve to chemically functionalize theisolated graphene sheets, thereby improving the chemical or mechanicalcompatibility of the graphene sheets with a matrix resin (e.g. epoxy) ina composite material.

The following examples serve to illustrate the best modes of practicefor the present invention and should not be construed as limiting thescope of the invention:

Example 1: Production of Isolated Graphene Sheets from MilledCoal-Derived Needle Coke Powder

Needle coke, milled to an average length <10 μm, was used as thestarting material. A needle coke sample (approximately 5 grams) wasplaced in a 100 milliliter high-pressure vessel. The vessel was equippedwith security clamps and rings that enable isolation of the vesselinterior from the atmosphere. The vessel was in fluid communication withhigh-pressure CO₂ by way of piping means and controlled or regulated byvalves. A heating jacket was wrapped around the vessel to achieve andmaintain the critical temperature of carbon dioxide. High-pressurecarbon dioxide was introduced into the vessel and maintained atapproximately 1,100 psig (7.58 MPa). Subsequently, the vessel was heatedto about 70° C. at which the supercritical conditions of carbon dioxidewere achieved and maintained for about 3 hours, allowing CO₂ to diffuseinto inter-graphene spaces and/or the amorphous zones between graphenedomains. Then, the vessel was immediately depressurized“catastrophically” at a rate of about 3 milliliters per second. This wasaccomplished by opening a connected blow-off valve of the vessel. As aresult, exfoliated graphene layers were formed, which were found to havean average thickness less than 1.0 nm. Various samples were collectedwith their morphology studied by SEM, TEM, and AFM observations andtheir specific surface areas measured by the well-known BET method. Thespecific surface area of the produced graphene sheets are typically inthe range from 735-920 m²/g, indicating that a majority of the graphenesheets being single-layer graphene, consistent with the microscopyresults.

A small amount of graphene sheets was mixed with water andultrasonicated for 15 minutes to obtain a suspension, which was thencast onto a glass surface to produce a thin film of approximately 90 nmin thickness. Based on a four-point probe approach, the electricalconductivity of the graphene film was found to be 3,965 S/cm.

Comparative Example 1a: Concentrated Sulfuric-Nitric Acid-IntercalatedNeedle Coke Particles

One gram of milled needle coke powder as used in Example 1 wereintercalated with a mixture of sulfuric acid, nitric acid, and potassiumpermanganate at a weight ratio of 4:1:0.05 (graphite-to-intercalateratio of 1:3) for four hours. Upon completion of the intercalationreaction, the mixture was poured into deionized water and filtered. Thesample was then washed with 5% HCl solution to remove most of thesulfate ions and residual salt and then repeatedly rinsed with deionizedwater until the pH of the filtrate was approximately 5. The dried samplewas then exfoliated at 1,000° C. for 45 seconds. The resulting NGPs wereexamined using SEM and TEM and their length (largest lateral dimension)and thickness were measured. It was observed that, in comparison withthe conventional strong acid process for producing graphene, thepresently invented supercritical fluid process leads to graphene sheetsof comparable thickness distribution, but much larger lateral dimensions(3-5 μm vs. 200-300 nm).

Graphene sheets were made into graphene paper layer using a well-knownvacuum-assisted filtration procedure. The graphene paper prepared fromhydrazine-reduced graphene oxide (made from sulfuric-nitricacid-intercalated coke) exhibits electrical conductivity values of11-143 S/cm. By contrast, the graphene paper prepared from therelatively oxidation-free graphene sheets made by the presently inventedsupercritical fluid process exhibits a conductivity value of 1,650 S/cm.

Comparative Example 1b: Preparation of Pristine NGPs from NaturalGraphite Using CO₂ Supercritical Fluids

A natural graphite sample (approximately 5 grams) was placed in a 100milliliter high-pressure vessel. The vessel was equipped with securityclamps and rings that enable isolation of the vessel interior from theatmosphere. The vessel was in fluid communication with high-pressure CO₂by way of piping means and controlled or regulated by valves. A heatingjacket was wrapped around the vessel to achieve and maintain thecritical temperature of carbon dioxide. High-pressure carbon dioxide wasintroduced into the vessel and maintained at approximately 1,100 psig(7.58 MPa). Subsequently, the vessel was heated to about 70° C. at whichthe supercritical conditions of carbon dioxide were achieved andmaintained for about 3 hours, allowing CO₂ to diffuse intointer-graphene spaces. Then, the vessel was immediately depressurized“catastrophically” at a rate of about 3 milliliters per second. This wasaccomplished by opening a connected blow-off valve of the vessel. As aresult, exfoliated graphene layers were formed, which were found tocontain pristine NGPs with an average thickness of approximately 10 nm.

A small amount of NGPs was mixed with water and ultrasonicated for 15minutes to obtain a suspension, which was then cast onto a glass surfaceto produce a thin film of approximately 89 nm in thickness. Based on afour-point probe approach, the electrical conductivity of the NGP filmwas found to be 909 S/cm.

Example 2: Repeated Intercalation and Exfoliation with CO₂ SupercriticalFluids

Portion of graphene sample produced in Example 1 was removed from thepressure vessel. The remaining graphene sample was subjected to anothercycle of supercritical CO₂ intercalation and de-pressurizationtreatments (i.e., the above procedures were repeated with a pressure of7.5 MPa and temperature 65° C.), yielding mostly single-layer graphene.

Example 3: Preparation of Pristine Graphene from Coal-Derived NeedleCoke Using CO₂ Supercritical Fluids Containing a Surfactant

Another graphene sample was prepared under essentially identicalsupercritical CO₂ conditions as in Example 1, with the exception that asmall amount of surfactant (approximately 0.05 grams of Zonyl® FSO) wasmixed with 5 grams of needle coke before the mixture was sealed in thepressure vessel. The resulting NGPs have a surprisingly low averagethickness, 0.8 nm. After the pressurization and de-pressurizationprocedures were repeated for one more cycles, the resulting NGPs weremostly single-layer.

Example 4: Production of Isolated Graphene Sheets from Milled PetroleumNeedle Coke Powder

Needle coke, milled to an average length <10 μm, was used as thestarting material. The dispersing agents selected include melamine,sodium (ethylenediamine), and hexamethylenete-tramine. Approximately 5grams of petroleum needle coke were placed in a high pressure vessel,which was supplied with CO₂ gas through pipe means as in Example 1. Thepressure at approximately 8.5 MPa was maintained while the vessel washeated to about 70° C. to achieve a supercritical condition of carbondioxide. This intercalation process was conducted for about 1 hour,followed by a sudden depressurization to the ambient pressure. Theresulting expanded/exfoliated structure after the initial cycle wasinvestigated using AFM, SEM, and BET measurements. The NGPs obtainedwere found to have an average thickness of 1.3 nm. The intercalation andexfoliation steps were repeated for another cycle and the resultinggraphene sheets are all single-layer or double-layer.

Example 5: Graphene Sheets from Milled Lignite Coal Powder

In one example, samples of two grams each of lignite coal were milleddown to an average diameter of 25.6 μm. The powder samples weresubjected to ethanol-based supercritical fluid treatments: 514° K and6.4 MPa, followed by rapid de-pressurization. The resulting graphenesheets exhibit a thickness ranging from single-layer graphene sheets to3-layer graphene sheets based on SEM and TEM observations

Example 6: Production of Isolated Graphene Sheets from Anthracite Coal

Taixi coal from Shanxi, China was used as the starting material for thepreparation of isolated graphene sheets. The raw coal was ground andsieved to a powder with an average particle size less than 200 μm. Thecoal powder was further size-reduced for 2.5 h by ball milling. Thediameter of more than 90% of milled powder particles is less than 15 μmafter milling. The raw coal powder was treated with hydrochloride in abeaker at 50° C. for 4 h to make modified coal (MC), and then it waswashed with distilled water until Cl⁻ no was detected in the filtrate.The modified coal was heat treated in the presence of Fe to transformcoal into graphite-like carbon. The MC powder and Fe₂(SO4)₃[TX-de:Fe₂(SO4)₃=16:12.6] was well-mixed by ball milling for 2 min, andthen the mixture was subjected to catalytic graphitization at 2400° C.for 2 h under argon.

The coal-derived powder samples were subjected to methanol-basedsupercritical fluid treatments under the conditions of 513° K and 8.2MPa, followed by rapid de-pressurization. The resulting graphene sheetsexhibit a thickness ranging from single-layer graphene sheets to 5-layergraphene sheets based on SEM and TEM observations.

Example 6: Production of Isolated Graphene Sheets from Bituminous Coal

In an example, 300 mg of bituminous coal was placed in a pressurevessel, which was then subjected to a CO₂ supercritical fluidintercalation for 1 h, followed by rapid pressure release. The productwas poured into a beaker containing a mixture of water and ethanol.After purification, the solution was concentrated using rotaryevaporation to obtain solid humic acid sheets.

A curing agent, aliphatic amine (EPIKURE 3223), was mixed with the humicacid powder at a 1:1 weight ratio. Epoxy resin (EPON 828) was then mixedwith the curing agent-coated humic acid sheets at a ratio of 12 parts ofcuring agent with 100 parts of epoxy resin and cured at 60° C. for 24hours to obtain an graphene-like nanocomposite.

Example 7: Intercalation and Exfoliation of Leonardite Coal with aSupercritical Fluid Containing SO₃

SO₃ vapor was generated by adding and heating 10 g of fuming sulfuricacid into a reactor. The SO₃ vapor was passed through a column in which10 g of leonardite was packed for receiving SO₃. After exposure ofleonardite to SO₃ for one hour, the treated leonardite sample was placedin a pressure vessel. The vessel was supplied with CO₂ gas through pipemeans as in Example 1. The pressure at approximately 8.5 MPa wasmaintained while the vessel was heated to about 70° C. to achieve asupercritical condition of carbon dioxide. Presumably SO₃ was dissolvedin supercritical CO₂. This CO₂/SO₃ intercalation process was allowed toproceed for about 3 hours, followed by a sudden depressurization to theambient pressure.

Upon completion of the procedure, the vessel containing NGPs wasslightly heated at about 60° C. for about 15 minutes to remove excessiveamount of SO₃ condensed on the surface of the NGPs, and the separatedSO₃ was recovered and absorbed into the sulfuric acid in the reactor.SO₃-treated NGPs were washed with water and filtered. Surprisingly,SO₃-treated NGPs were found to be readily dispersible in water. Itappears that SO₃ has slightly sufonated NGPs, imparting desirablefunctional groups thereto.

It is of significance to note that SO₃, O₃ (ozone) and O₂ are but a fewexamples of reactive species that can be included in a supercriticalfluid for exfoliating and, essentially concurrently, functionalizingNGPs.

Example 8: Production of Graphene Thin Films

In the aforementioned Example 1, a desired amount of slurry containingfully separated graphene sheets dispersed in water was made into a wetfilm on a PET substrate (poly ethylene terephthalate) using a slot-diecoater. The wet film was then heated at 85° C. for 2 hours andmechanically compressed to become a flexible graphene film. Theresulting flexible graphene film, having not been previously exposed toa significant oxidation environment, exhibits an electrical conductivitytypically from 3,000 to 7,500 S/cm and thermal conductivity from 650 to1,200 W/mK. By contrast, commercially available flexible graphitesheets, prepared by thermal exfoliation of GICs and re-compression ofexfoliated graphite, exhibit an electrical conductivity typically lowerthan 1,200 S/cm and thermal conductivity lower than 500 W/mK. Also,graphene films produced by the conventional Hummer's method and thesubsequent slot-die coating under comparable conditions show anelectrical conductivity of 1,500-3,000 S/cm and thermal conductivity400-600 W/mK.

In summary, the presently invented process is superior to many prior artprocesses in several aspects:

-   -   1) Prior art processes based on graphite intercalation/oxidation        and high-temperature exfoliation did not allow for a good        control over the oxygen content of the resulting GO or NGP        platelets. The presently invented process is capable of        producing pristine NGPs that have never been exposed to        oxidation.    -   2) In another commonly used prior art approach, the graphite        oxide dispersed in an aqueous solution was reduced with        hydrazine, in the presence of a polymer, such as poly (sodium        4-styrenesulfonate). This process led to the formation of a        stable aqueous dispersion of polymer-coated graphene platelets.        In some applications of NGPs, however, a polymer coating may be        undesirable. Furthermore, the commonly used reducing agent,        hydrazine, is a toxic substance.    -   3) Conventional processes of preparing GO nanosheets that        included chemical exfoliation typically were extremely tedious.        Such a long process is not amenable to the mass production of        conductive graphene sheets.    -   4) The presently invented process is capable of producing NGPs        with no or little impurity. The process can obviate the need for        washing and rinsing the platelets (which was required in the        prior art solution approach to the exfoliation of GO and/or        subsequent chemical reduction). The presently invented process        is fast and environmentally benign.    -   5) The presently invented process is capable of producing        ultra-thin NGPs, including those that are single graphene        sheets.    -   6) This process allows for concurrently attaching a desirable        functional group to the resulting NGPs (e.g., by simply        introducing a desirable chemical species, such as SO₃, into the        supercritical fluid). This is a powerful approach to varying the        dispersibility or solubility of NGPs in a solvent.

The invention claimed is:
 1. A process for producing a dispersion ofisolated graphene sheets without purification, said process comprising:(a) exposing coal or coke powder and a liquid medium to a supercriticalfluid at a first temperature and a first pressure for a first period oftime in a pressure vessel and (b) depressurizing said supercriticalfluid at a fluid release rate to produce said isolated graphene sheets;wherein said coal or coke powder is selected from the group consistingof petroleum coke, coal-derived coke, mesophase coke, synthetic coke,leonardite, anthracite, lignite coal, bituminous coal, natural coalmineral powder, and combinations thereof.
 2. The process of claim 1further comprising a step of a shearing said coal or coke powder by aprocess selected from wet milling, ball milling, rotating bladeshearing, ultrasonication, and combinations thereof.
 3. The process ofclaim 1, further comprising a procedure of repeating step (a) and step(b) that includes (i) subjecting said isolated graphene sheets to asupercritical fluid at a second temperature and a second pressure for asecond period of time in a pressure vessel and then (ii) depressurizingsaid fluid at a fluid release rate.
 4. The process of claim 3, whereinsaid second temperature differs from said first temperature or saidsecond pressure differs from said first pressure.
 5. The process ofclaim 1, wherein said supercritical fluid comprises a material selectedfrom the group consisting of carbon dioxide, water, hydrogen peroxide,methanol, ethanol, acetone, methane, ethane, propane, ethylene,propylene, nitrous oxide, ozone, sulfur oxide, and combinations thereof.6. The process of claim 1, wherein said supercritical fluid furthercomprises a reactive chemical group and said step of exposure to asupercritical fluid imparts said reactive chemical group to saiddispersed graphene sheets.
 7. The process of claim 1, wherein said firstperiod of time is from 10 minutes to 24 hours.
 8. The process of claim1, wherein said step of depressurizing has at a rate of pressure releasebetween 0.1 and 5.0 milliliters per second.
 9. The process of claim 1,wherein said liquid medium is selected from water, alcohol, or acetone.10. The process of claim 1, wherein said liquid medium further comprisesa material selected from the group consisting of dispersing agent, anorganic solvent, a monomer, an oligomer, a polymer solution, andcombinations thereof.
 11. The process of claim 1, wherein said liquidmedium further comprises a material selected from the group consistingof sodium hexametaphosphate, sodium lignosulfonate, poly (sodium4-styrene sulfonate), sodium dodecyl sulfate, sodium sulfate, sodiumphosphate, sodium sulfonate, melamine, ammonium sulfate, sodium(ethylenediamine), tetraalkylammonium, ammonia, carbamide,hexamethylenetetramine, organic amine, pyrene, 1-pyrenecarboxylic acid,1-pyrenebutyric acid, 1-pyrenamine, and combinations thereof.
 12. Theprocess of claim 1 wherein said pressure vessel supercritical fluidfurther comprises a coating agent.
 13. The process of claim 12, whereinsaid coating agent is selected from a monomer, a prepolymer or oligomer,a polymer, a resin, a curing agent, or a combination thereof.
 14. Theprocess of claim 12, wherein said coating agent is selected from thegroup consisting of poly-(dimethyl siloxane),poly-(tetrafluoroethylene-co-hexafluoropropylene),poly-(perfluoro-propylene oxide), poly-(diethyl-siloxane),poly-(dimethyl silicone), poly-(phenylmethylsilicone),perfluoroalkylpolyethers, chloro-trifluoro-ethylene, andbromotrifluoroethylene.
 15. The process of claim 12, wherein said coalor coke powder comprises 25 to 85 weight percent and the coating agentcomprises 15 to 75 weight percent of material input into the process.16. The process of claim 1 further comprising a step of coating saidgraphene-containing dispersion onto a solid surface to form a wet film,and drying said wet film, to form a dried graphene film.
 17. The processof claim 16 further comprising a step of compression.
 18. A process forproducing a dispersion of isolated graphene sheets without purification,said process comprising: (a) exposing coal or coke powder and a monomeror oligomer to a supercritical fluid at a first temperature and a firstpressure for a first period of time in a pressure vessel and (b)depressurizing said supercritical fluid at a fluid release rate toproduce a dispersion of isolated graphene sheets; wherein said coal orcoke powder is selected from the group consisting of petroleum coke,coal-derived coke, mesophase coke, synthetic coke, leonardite,anthracite, lignite coal, bituminous coal, natural coal mineral powder,and combinations thereof.
 19. The process of claim 11, furthercomprising a step of polymerization to form a polymer composite.
 20. Acomposite material containing graphene sheets produced by the method ofclaim 18.